1. Field of the Invention
The invention relates to an image composition method in which a first shift value for a mutual shift of successive sub-images is derived from first portions of sub-images and respective first portions of successive sub-images are merged, on the basis of the first shift value so as to form a combined first portion.
2. Description of Related Art
A method of this kind is known from European Patent Application EP 0 655 861.
The composition of an image from sub-images is generally used to form a composite image of an elongate scene which is too long to be reproduced in one operation. In medical X-ray diagnostics such a situation occurs notably for peripheral X-ray angiography where X-ray images are formed of blood vessels in the limbs, but also, for example, when an image of the spinal column is made. Using a contemporary X-ray examination apparatus it is difficult or even impossible to form an X-ray image of the complete region of an arm or a leg of a patient to be examined in one exposure. A number of successive X-ray images of portions of the region to be examined are formed, which images together cover the entire region. An X-ray image intensifier forms optical images from the X-ray images and an image pick-up apparatus, for example a television camera, derives image signals from the optical images. The image signals represent the sub-images used to form the composite image.
The known method includes a procedure for deriving a mutual shift of successive sub-images from correlation between pixel values of overlapping portions of successive sub-images. Upon combination of the sub-images, pixel values of mutually overlapping portions of successive sub-images and those relating to the same position in the elongate scene are interpolated so as to form pixel values of the composite image. The known method is successful notably in preventing mutual differences in brightness values in overlapping portions of successive sub-images from introducing disturbances in the composite image. However, in the case of parallax in the sub-images, the known method is not adequate in counteracting resultant disturbances in the composite image. Notably when X-ray images are formed, shadow images are produced in which objects situated at different distances from the X-ray source are imaged in the same image plane. Due to parallax, differences occur in the images of objects situated at different distances from the X-ray source and in different sub-images. In the known method such differences still cause disturbances in the composite image of the elongate scene which shows, for example the vascular system in a leg of the patient.